Here's the explanation for why we are not using the priority system for 2.0
"Based upon feedback received regarding WCAG 1.0, the current 2.0 version of
the guidelines are not divided up according to priority. The reason for
this is that each success criteria, regardless of priority category that it
would fall into, could make the critical difference between accessibility
and inaccessibility for some group of users. Given this, it is impossible
to make some checkpoints more important than others without implying that
some users are more important than others. Thus, the guidelines and
checkpoints have been reorganized based upon the degree to which each
checkpoint doesn't prescribe the default presentation or expression of the
content. "
David MacDonald
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Access Empowers People...
...Barriers Disable Them
<http://www.eramp.com/> www.eramp.com